The lateral gastrocnemius and plantaris tendons are each labelled with two 1 mm dia lead balls separated by about 2.0 cm in a series of small dogs. Bone screws attached to the caudal aspect of the tribia serve as a length standard in the same plane as the tendon markers. As the dog runs on a treadmill, the tendon stretch is recorded by high-speed x-ray cine. After all experiments, the tendon force-length relation is calibrated in vitro, allowing a direct calculation of muscle force, tendon strain, and energy stored in muscle and tendon during running.